A combination of analytical techniques, including X-ray photoelectron spectroscopy (XPS), solid state 13C nuclear magnetic resonance (NMR) spectroscopy, and supercritical fluid extraction/mass spectrometry (SFE/MS), were used to characterize the detailed composition and structure of coke formed on catalyst in the fluid catalytic cracking (FCC) process. By characterizing coke samples from a series of designed FCC experiments, the effects of conversion on coke composition were systematically studied. SFE is shown to be an effective technique for removing low molecular weight coke molecules from the catalyst. When combined with mass spectrometry, the technique provided molecular level information of the extracted coke species. The coked catalysts were directly analyzed by XPS and NMR to obtain information relevant to surface and bulk coke structures, respectively. The study revealed the presence of two types of nitrogen-based coke and showed that N distributions were strongly affected by FCC conversion level. The study also suggests that most nitrogen-containing coke is formed in the earlier stages of cracking while hydrocarbons are the primary contributors to coke yield in the later stages of cracking. The aromaticity of coke remains fairly constant at high conversions.

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A mechanistic model for the catalytic cracking of n-heptane was developed using a novel mechanism-based lumping scheme that exploits the chemical similarities within reaction families. The formal application of 13 reaction family matrices, which correspond to the 11 reaction families in the model, to the matrix representations of the reactants and derived products generated 70 species, 235 elementary steps and 70 ordinary differential equations. The reaction family concept was further exploited to constrain the kinetics within each reaction family to follow a quantitative structure/reactivity Polanyi relationship. Ultimately, four Polanyi relationship parameters and one catalyst specific parameter were optimized using experimental data obtained from the cracking of n-heptane at 500 °C over HZSM-5 with a Si/Al ratio of 21.25. The model correlations were excellent, as were the a priori predictions of experimental results at 450 and 550 °C with an HZSM-5 Si/Al ratio of 21.25 and at 500 °C with HZSM-5 Si/Al ratios of 35.25 and 63.5. The thus validated model was then used to probe the controlling elementary steps of n-heptane cracking. Carbonium ion cracking, β-scission, and hydride transfer were the kinetically significant reactions.

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The effect of hydrogen transfer reactions on ZSM-5 additive in a fluid catalytic cracking system was investigated. Microactivity testing of 4 wt % of ZSM-5 additive on 10 different commercial FCC gas oil feeds showed that the percentage increases of propylene and butylene due to ZSM-5 additive decreases with increasing naphthenes in the feed, due to the high propensity of naphthenes to donate hydrogen. The hydrogen transfer reactions tend to saturate the olefins or olefin precursors which otherwise could be cracked by ZSM-5 into the desired LPG olefins (propylene and butylene). The effect is also confirmed by a model compound study with adamantane. A high content of rare earth on zeolite provides higher acid site density, thus promoting hydrogen transfer. The effectiveness of ZSM-5 is the highest with the catalyst having the lowest acid site density or lowest rare earth level on zeolite.

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